Ventilation devices adapted to be installed in a roof of a building are well known in the art. These devices are particularly important in cool weather when the interior temperature of the building is warm and the ambient outside temperature is cool. Under these conditions, moisture in the ambient atmosphere tends to condense on top of the roof. Moisture build-up on the roof not only results in degradation of the roof structure but also tends to have a deleterious effect on materials or apparatus stored beneath the roof. Ventilation devices are therefore commonly employed to provide an outlet for moisture through the roof.
Ventilation devices are also employed to prevent accumulation of moisture in insulation beneath the roof since moisture laden insulation drastically reduces insulation efficiency.
Still another purpose of ventilator devices is to allow escape of stale air.
Ventilation devices of the type described above are commonly constructed of metal materials, especially aluminum which are well suited to withstand exposure to the elements without rusting, discoloration, etc. The portions of metal ventilators that project outwardly beyond the roof are apt to be considerably cooler than the air in the interior space of the enclosure. As a result, when the moisture laden air contacts the inside surfaces of the vent, it immediately starts to cool thereby resulting in moisture which is condensed and collects from the interior surfaces of the ventilator. This phenomena reduces venting efficiency and in many cases the amount of moisture that may collect is sufficient to run downwardly back into the building.
U.S. Pat. No. 4,214,513 issued to James Ballard on July 29, 1980 discloses a ventilator device well adapted for use with the present invention. The ventilator shown in the Ballard patent includes a flat, annular base adapted to be attached to the roof and an upwardly extending tubular portion having a cap which prevents the introduction of rain or the like directly through the vent into the building. With a vent of this type, particularly during the colder seasons of the year in northern climates, there is a considerable difference in the temperature surrounding the exterior portion of the vent compared to the temperature of the air in the building. As a result, the moisture within the building, in the form of water vapor, is quickly cooled when it strikes the sidewalls of the cylindrical portion of the ventilator whose exterior surface is surrounded by the ambient cold air. The condition results in a considerable amount of moisture on the interior walls of the ventilator, which moisture is drawn by gravity back into the enclosure.
Another problem associated with prior art ventilator devices in addition to the fact that they are inclined to produce condensation involves the fact that they allow moisture in the ambient environment to enter the building. This phenomena is particularly prevalent during the evening hours when the roof structure cools. While water vapor which is present within an attic, for example, may well move outwardly during the daylight hours through the ventilator, it is often found that under cooler conditions, moisture is drawn back into the building through the same ventilator.
U.S. Pat. No. 3,984,947 issued to Patry October, 1976 discloses an example of a construction for preventing the backflow of moisture into a vented air. According to the patent, there is provided a flexible diaphragm which will deform under the influence of the air pressure experienced on its opposite sides. If the pressure within the building is higher than that outside, the diaphragm will flex and expose venting holes or openings through which the air can escape. On the other hand, if the pressure on the opposite side of the diaphragm is greater than that within the building, the diaphragm will close and there will be no passage for moisture to move inwardly pass the diaphragm into the building.